The prosthetic replacement of hip joints, either the acetabular component implanted in the patient's pelvis or the femoral component implanted in the femur, or both, is now widely practiced to replace degenerated natural hip joints. Prosthetic hip joints have evolved over the years from early, relatively crude models to current prostheses which closely duplicate the functions and motions of a natural joint. As a result, prosthetic hips have provided patients with increasing comfort, freedom of motion and an ability to lead nearly normal lives.
Although there have been problems with excessive wear between components of prosthetic devices which move with respect to each other, by and large the fixation of the prosthetic components to the patient's bone structure did and continues to represent the greatest difficulty. Early attempts to incorporate large fenestrations or openings in the implant components, which were thought to mechanically lock the implants to the bone by promoting the growth of bone through such openings, were soon discarded because they proved unsuccessful. With the event of tissue compatible acrylic cement, implants were increasingly cemented to the bone and this practice continues to be widely followed because, at least in the short term, it has proved to be highly successful.
However, the longevity of cemented implants suffers primarily as a result of the differences in the moduli of elasticity at bone/cement and the cement/implant interfaces. For certain patients a loosening of the implant takes place after a number of years of other wise successful use. This can be painful and frequently requires the replacement of the implant which is burdensome, expensive and can incapacitate the patient for significant periods of time.
In the recent past, attempts have again been made to enhance the longevity of prosthetic implants by supplementing the cement bond with at least a degree of direct bone-implant interlocking. This has been accomplished by providing porous implant surfaces which contact the bone tissue so that, after a typical ingrow period of several weeks, bone tissue grows into the pores and thereby forms a firm, mechanical connection. To achieve such bony ingrowth, it is necessary that any relative movements between the bone and the porous implant surfaces are prevented. Currently, cement continues to provide the necessary fixation of the implant.
In spite of the improvements and advances that have taken place, the fixation of the implant to the surrounding bone structure remains the source of most implant failures. It is believed that this results from both failures of the implant-bone bond and the stresses generated between the implant, the bone and/or the cement.
For example, femoral components of hip implants typically include elongated stems which extend into the medullary cavity of the femur and, depending on the particular technique employed, are bonded to the surrounding bone structure and/or a bone ingrowth into porous implant surfaces is attempted. The transfer of forces from the implant to the bone generates shear stresses which are not readily transferred, which tend to weaken the interface and, over time, are likely to destroy the connection.
In addition, the load carrying structure of the femur is unnaturally stressed because the transfer of forces takes place over the entire length of the implant stem extending deep into the medullary cavity. In contrast, the normal load transfer to the femur is from the top. As a consequence, the absence of proper stressing of the femur from the top when conventional femoral implants are utilized leads to stress shielding at the top and a resultant bone resorption in this upper region which, in time, can lead to implant and/or bone failures.
The acetabular components of prior art prosthetic hips are similarly deficient. First, the fixation of the acetabular cups within corresponding sockets in the patient's hip is difficult because of their semispherical shape. In almost all instances, the cups are bonded to the pelvis with cement which, at various points over the exterior surfaces of the cups, is subjected to compression, shear or both. Over time, such unequal stressing of the bond is likely to lead to mechanical failure.
Additionally, the semi-spherical shape of the acetabular cups makes it difficult to properly locate the cup in the socket and fix it. Attempts have been made to provide such cups with spikes or screws to mechanically lock them in position. However, as these are driven into the bone an uncontrolled and undesirable repositioning of the cups is almost impossible to prevent.
As with femoral components, attempts have been made to improve the fixation of acetabular hip joint components by forming them with porous exterior surfaces to promote bone ingrowth and thereby establish a mechanical interlock between the patient's natural bone and the implant. Since such bone ingrowth requires immediate rigid fixation of the implant, cement continues to be used widely for initially securing the implant to the bone. Moreover, in the past it was thought desirable to attain bone ingrowth over as large a surface area of the acetabular component as possible. This results in bone ingrowth that is partially subjected to shear stresses along the sides of the cup. This makes it not only difficult to obtain bony ingrowth along the sides but can be harmful. If a bony ingrowth is obtained along the sides where shear stresses occur, there may occur an abnormal transference of stress to the top of the cup where most loading occurs in the natural state. The result will be stress shielding with resultant bone resorption about the top of the cup over a period of years, leading to potential mechanical failure of the device.
U.S. Pat. Nos. 4,068,324 and 3,840,904 are examples of recent developments and improvements in the construction of femoral and acetabular hip joint components.